The European Union (EU) is increasingly dependent on the import of not only primary energy sources, but also of industrial raw materials. The EU is therefore more exposed and vulnerable than other states to the effects of market distortion. Some of these industrial primary raw materials are used in the manufacture of so-called high technology products. The products in question are utilized in, among others, environmental technology solutions, to promote the improvement of energy efficiency and the reduction of greenhouse gas emissions.
In 2010, the European Commission analyzed the economic importance and availability risk of a total of 41 raw materials used by industry. Fourteen of the minerals and metals analyzed were deemed critical to the industrial activity of the European Union, because they have a significant economic effect on key sectors, or their availability and replacement contain significant risks. The raw materials classified as critical are antimony, indium, beryllium, magnesium, cobalt, niobium, calcium fluoride, the metals of the platinum group, gallium, the rare earth elements (lanthanums), germanium, tantalum, graphite, and wolfram.
Every year, about one million tonnes of waste are created in Finnish power plants. The waste is mostly ash arising from combustion and sulphur removal. The ash is either so-called bottom ash or fine particle fly ash collected from flue gas filters. The ash typically contains mainly incombustible minerals, silicates, and possibly also heavy metals. Most of this ash, about 60%, is used as various earthworks, for example, in field structures and as a filler in landfill structures, as well as a batching material in concrete and cement, for example, as a raw material in cement and in building boards. These exploitable ashes are typically utilized for such purposes in the state in which they leave the power plant. Most of these exploitable ash wastes (about 55%) are generated from coal burning.
The low degree of utilization has partly been due to the relatively cheap final disposal costs and the statutory waste status of ash, as well as the tight content restrictions in, for example, fertilizer and earthwork uses. However, changing tax procedures and steadily rising transport costs have placed a continually increasing cost pressure on power plants in terms of ash treatment.
In waste exploitation, the point of departure is to meet statutory obligations. Attempts have been made to use legislation to facilitate the use in earthworks of bottom and fly ash from the combustion of coal, peat, and wood-based material. However, the quality of the ashes must be defined and monitored. By also limiting the thickness of a final disposal structure, the aim has been to prevent the creation of uncontrolled sorting areas. For example, fly ash can be consolidated if water is added to it and it is compacted. Fly ash can then be used, for example, as structural layer in a road.
Most of the ash from mixed combustion is formed in fluid bed combustion power plants. The quality of wood ash also varies between the different parts of a tree. For example, the metals contents relative to energy content are greater in the bark and branches than in the trunk. The element contents of the ground also vary according to time and place, which affects the quality of the ash. When trees and plants grow, they absorb minerals and elements along with water from the ground, which enrich the structures of the trees and plants during growth. Indeed, it can be assumed that plants manifest the geology of the area in which they grow, and that variations in the element contents of the ground can be detected from the composition of the ash.
Quite a large number of solubility studies exist for the fly ash of coal. The emphasis of these studies is generally the solubility of specific harmful substances. The solubility of other metals from the fly ash of coal has been shown to be quite small. The solubility properties of the ash from mixed combustion generally correspond to the solubility of ash formed from the combustion of coal and peat.
The amount of biofuels used in energy production is increasing due to the aims and objectives of climate and energy policies. The most important reasons for the increase in the use of biofuels are the EU's statutory greenhouse gas reduction goals through the year 2020 and the aim of increasing renewable energy. The reduction goal for greenhouse gases is 20% of the year 1990 level, and the goal for increasing renewable energy is 20% of total energy consumption compared to the year 2005 level. The increasing use of biofuels in power plants changes not only the combustion event but also the composition of the ash that is created.
There are several methods, most of which have been developed to make the processing of ashes suitable for disposal in a landfill. Dry ash can be air classified, in which the ash is divided into various fractions on the basis of particle size and specific weight. Most soluble substances and heavy metals exist in small particles, which can be separated by air classification. Correspondingly, soluble substances can be separated using water or acid washing. However, washing increases processing costs and creates waste water. The solubility properties of ash can also be affected by storage. When ash ages, it reacts with air, changing its solubility. Heavy metals can be removed by thermal methods. However, heating processes consume a lot of energy and do not completely purify the ash.
Finnish patent number 101572 discloses a method of stabilizing fine ash into larger ash particles. However, the disclosed method requires a combustion plant of a specific type. In addition, the method is unsuitable for processing fly ash, which is removed only in the final stage of the combustion process. The use of fly ash for earthworks is problematic due to its capillary structure. In practice, a layer formed of fly ash is susceptible to frost heave even when compacted.
Japanese patent application number 2007321239 discloses the recovery of copper from fly ash. In the disclosed method, the fly ash is treated with additives and the mixture is processed at a high temperature. The method is suitable for only a limited number of elements and requires a great deal of energy, while providing only a modest yield.